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901 vs. 921

Posted: Fri Oct 06, 2017 9:29 am
by analogmonster
Hi owners,

I read several times that the 901 sounds "better" or "smoother" than the 921. As a square is a square and a saw is a saw and so on: are there any measurable differences in the waveforms? An audible difference should be visible in osc pictures.
I noticed some waveform anomalies when I cloned the 901, but I attributed this to component differences, but perhaps owners of the original saw similar things or other differences in their osc pictures?
See also waveform pics at http://www.analog-monster.de/mmt901_en.html

Thanks

Re: 901 vs. 921

Posted: Fri Oct 06, 2017 2:51 pm
by ualslosar
The 901's synch better together than the 921's, not that the waveforms are any different. I believe this has to do with the internal architecture of the 901s vs the 921s, from what i understand.

Re: 901 vs. 921

Posted: Fri Oct 06, 2017 3:08 pm
by MC
The waveforms on the 901 are more distorted. The rising/falling edges of the square waves are slower, the slope of the ramp is more curved, and the triangle has a slight discontinuity in the peaks due to the UJT used in the waveshaping circuit. The audible differences are subtle but they are there. In fact, the Voyager simulates the curved ramp and the triangle discontinuity.

Whether they sound "better" is subjective. The vintage 901A driver wasn't stable or accurate throughout the five octave keyboard range (due to the limited temperature control available with the transition knees of the CA3019 diode bridge), but I don't know if the reissue 901A has resolved that.

Re: 901 vs. 921

Posted: Fri Oct 27, 2017 11:32 am
by robotmakers
The magic of the 901 modular VCO set is that the 901Bs share one exponential converter in the 901A. This means that when the 901Bs are set to detune slightly from each other, the RATE of beating (i.e. the offset in Hz) remains constant throughout the range of the keyboard. Technically, each 901B is a linear response VCO.

On a 921 VCO set (and indeed with virtually all other VCOs), each 921B has its own exponential converter. This means that when one 921B is set to detune against another, the RATIO of the frequencies remains constant. This means that the rate of beating doubles with each octave you go up the keyboard. Technically, each 921B is an exponential response VCO.

So, with the 901s, you can set the detune beating to remain constant (and pleasantly slow) across the keyboard range. This gives the impression of "fatness". With other VCOs like the 921, the beating gets faster as you go up the keyboard. Less "fat".

Re: 901 vs. 921

Posted: Mon Dec 11, 2017 5:26 pm
by till
robotmakers wrote:The magic of the 901 modular VCO set is that the 901Bs share one exponential converter in the 901A. This means that when the 901Bs are set to detune slightly from each other, the RATE of beating (i.e. the offset in Hz) remains constant throughout the range of the keyboard. ...

You nailed it! This is something I had discovered after I tried the 901 clone oscillator bank be Synth-Werk from Munich/Germany.

Re: 901 vs. 921

Posted: Fri Jan 19, 2018 10:42 pm
by thedangore
MC wrote:The waveforms on the 901 are more distorted. The rising/falling edges of the square waves are slower, the slope of the ramp is more curved, and the triangle has a slight discontinuity in the peaks due to the UJT used in the waveshaping circuit. The audible differences are subtle but they are there. In fact, the Voyager simulates the curved ramp and the triangle discontinuity.

Whether they sound "better" is subjective. The vintage 901A driver wasn't stable or accurate throughout the five octave keyboard range (due to the limited temperature control available with the transition knees of the CA3019 diode bridge), but I don't know if the reissue 901A has resolved that.


I can say with confidence that the issue has not been resolved in the new reissue units. Still the same old quirky tuning problems over the octave range, though certain parameters with the seem to be more workable than others for keeping tune. 901s most definitely have a different texture than 921s. And it is definitely better for certain things though I still find myself using 921s for other things just as much (like using the 921a for its pwm capabilities).

Re: 901 vs. 921

Posted: Sat Jan 20, 2018 7:45 am
by noddyspuncture
thedangore wrote:I can say with confidence that the issue has not been resolved in the new reissue units.


Are there reissued 901's...?
I thought all the reissue's contain the 921's...!?

Re: 901 vs. 921

Posted: Sat Jan 20, 2018 12:30 pm
by thedangore
noddyspuncture wrote:
thedangore wrote:I can say with confidence that the issue has not been resolved in the new reissue units.


Are there reissued 901's...?
I thought all the reissue's contain the 921's...!?



Hey Noddy,

Definitely yes! Recently (Aprilish 2017?) Moog reissued the IIIC which contains all the 901 oscillators - nine 901bs and one 901 oscillator! And Beautiful they are! HUGE sound. But all that same quirky stuff is still there as I'm sure you're well aware of. I was fortunate enough to grab #10/25

Re: 901 vs. 921

Posted: Sat Jan 20, 2018 1:01 pm
by EMwhite
thedangore wrote:I was fortunate enough to grab #10/25


I would love to see some pics and videos of it in action. Such a shortage (obviously) of this. Despite Moog's introductory video (which was excellent), haven't seen much.

Re: 901 vs. 921

Posted: Sat Jan 20, 2018 7:56 pm
by noddyspuncture
EMwhite wrote:
thedangore wrote:I was fortunate enough to grab #10/25


I would love to see some pics and videos of it in action. Such a shortage (obviously) of this. Despite Moog's introductory video (which was excellent), haven't seen much.


Same here... yes I'd like to see that also please. :D